. On this day in 1904 – December 20th – The Mount Wilson Solar Observatory in Los Angeles County, California was founded.
About fifteen years later, "solar" was dropped from the name as the 100-inch Hooker telescope came into use. Observing with it, Edwin Hubble found evidence that the Milky Way was just one galaxy in an expanding Universe.
. On this day in 2021 – December 21st – the winter solstice occurred at 15:58 UCT (Coordinated Universal Time), i.e., Greenwich Mean Time. It was 3:58 PM if you don't use a 24-four clock.
The winter solstice is is the shortest day of the year in the northern hemisphere, and the North Pole is at its maximum tilt away from the Sun. The southern hemisphere has its longest day and, the South Pole is at its maximum tilt towards the Sun.
. On this day in 1968 – December 24th – Apollo 8 entered lunar orbit.
The crew - Frank Borman, James Lovell & Wiliam Anders - were the first humans to view Earth as a whole planet, the first to orbit another world, and the first to see the far side of the Moon. They made a live Christmas Eve broadcast to the Earth as they circled the Moon.
. On this day in 1780 – December 26th – Mary Somerville was born in Jedburgh, Scotland.
Somerville was one of the leading lights of 19th century science. She was an able mathematician and she published books on developments in astronomy, physics and geography. Somerville was honored by several learned societies, even though, as a woman, she would not have been allowed to attend their meetings.
. On this day in 1571 – December 27th – Johannes Kepler was born near Stuttgart in Germany.
Using the data of Tycho Brahe, Kepler formulated the three laws planetary motion which were the foundation for our modern view of the Solar System. They could describe planetary orbits simply, but required the Sun - not the Earth - to be at the center of the Solar System.
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